Lower Hutt police are looking for two sets of body armour left on a roadside by mistake.
Inspector Mike Hill says a patrol vehicle was attending an incident on Saturday night and the boot of the car emptied to make room for seized firearms.
The bulletproof armour, costing $35,000, was left in St James Grove in the suburb of Boulcott.
When officers went back to retrieve the vests several hours later, they were gone.
An investigation has begun and the Independent Police Conduct Authority notified.
Police would like to hear from anyone who may have seen the vests or know of their whereabouts.